Measuring device for amount of charge of toner and image forming apparatus having the measuring device

Abstract

A measuring device for an amount of charge of toner including countered first and second electrodes for forming an electric field which accelerates toner particles, a voltage generator for applying a voltage across the first and second electrodes, a magnifying optical system for magnifying the image of toner particles which move between the first and the second electrodes, a light source for illuminating a vicinity of a focus of the magnifying optical system, a CCD sensor for receiving an optical image of the toner particles magnified by the magnifying optical system so as to convert the optical image into an image signal and a CPU for controlling a voltage from the voltage generator based upon the image signal so that the toner particles pass a prescribed position in the vicinity of the focus of the magnifying optical system and for obtaining an amount of charge of toner particles from a value of the voltage, and an image forming apparatus in which the measuring device is installed. The measuring device for an amount of charge of toner becomes compact, so it can be installed in an image forming apparatus.

What is claimed is: 
     
       1. A measuring device for an amount of charge of toner, comprising: countered first and second electrodes for forming an electric field which accelerates toner particles;   a voltage generator for applying a voltage across said first and second electrodes;   a magnifying optical system for magnifying an image of the toner particles which move between said first and second electrodes;   a light source for illuminating a vicinity of a focus of said magnifying optical system   a CCD sensor for receiving an optical image of the toner particles magnified by said magnifying optical system so as to convert the optical image into an image signal; and   control means for controlling a voltage from said voltage generator based upon the image signal so that the toner particles pass a prescribed position in the vicinity of the focus of said magnifying optical system and for obtaining an amount of charge of toner particles from a value of the voltage.   
     
     
       2. The measuring device for an amount of charge of toner as defined in claim 1 is provided with a first window and a second window, said first window preventing the toner particles from adhering to said light source, said second window preventing the toner particles from adhering to said magnifying optical system. 
     
     
       3. The measuring device for an amount of charge of toner as defined in claim 2, wherein said first and second windows are composed of a translucent substrate and a conductive coating which has been formed on opposite inner surfaces where the toner particles pass. 
     
     
       4. The measuring device for an amount of charge of toner as defined in claim 3, wherein said first electrode is connected to said second electrode through the conductive coating in said first window and through the conductive coating in said second window. 
     
     
       5. The measuring device for an amount of charge of toner as defined in claim 3, wherein said first electrode is connected with the conductive coating in said first window and said second electrode is connected with the conductive coating in said second window. 
     
     
       6. The measuring device for an amount of charge of toner as defined in claim 5, further comprising: a first switch means for connecting said first electrode with the conductive coating in said second window;   a second switch means for connecting said second electrode with the conductive coating in said first window; and   a voltage applying means for applying an alternating voltage to between said first and second electrodes.   
     
     
       7. The measuring device for an amount of charge of toner as defined in claim 3, wherein the translucent substrate in said first and second windows is composed of glass and the conductive coating is composed of SnO 2  and Cr. 
     
     
       8. The measuring device for an amount of charge of toner as defined in claim 3, wherein electric conductivity of the conductive coating is 1 kΩ/cm 2  order. 
     
     
       9. The measuring device for an amount of charge of toner as defined in claim 1, further comprising means for grounding said first and second electrodes in order to obtain mass of the toner particles from fall speed of the toner particles. 
     
     
       10. The measuring device for an amount of charge of toner as defined in claim 1, wherein the surfaces of said first and second electrodes are set so as to be level with each other so that said first and second electrodes can accelerate the toner particles in an opposite direction to gravitation. 
     
     
       11. The measuring device for an amount of charge of toner as defined in claim 10, wherein said control means controls a voltage generated from the voltage generator so that gravitation which is applied to the toner particles is eliminated. 
     
     
       12. The measuring device for an amount of charge of toner as defined in claim 1, wherein said CCD sensor is an area sensor. 
     
     
       13. The measuring device for an amount of charge of toner as defined in claim 12, wherein said light source is a semiconductor laser for emitting a light in a prescribed cycle. 
     
     
       14. An image forming apparatus, comprising: a photosensitive drum;   a developing unit for allowing charged toner to adhere to said photosensitive drum according to image information; and   a measuring device for measuring an amount of charge of toner which has been charged in said developing unit,   wherein said measuring device for an amount of charge of toner includes:   countered first and second electrodes for forming an electric field which accelerates toner particles;   a voltage generator for applying a voltage across said first and second electrodes;   a magnifying optical system for magnifying the image of toner particles which move between said first and second electrodes;   a light source for illuminating a vicinity of a focus of said magnifying optical system;   a CCD sensor for receiving an optical image of the toner particles magnified by said magnifying optical system so as to convert the optical image into an image signal; and   control means for controlling a voltage from said voltage generator based upon the image signal so that the toner particles pass prescribed position in the vicinity of the focus of said magnifying optical system and for obtaining an amount of charge of toner particles from a value of the voltage.   
     
     
       15. The image forming apparatus as defined in claim 14, further comprising: an extraction electrode for extracting toner charged in said developing unit; and   a pump for carrying the toner extracted by said extraction electrode to between said first and second electrodes.   
     
     
       16. The image forming apparatus as defined in claim 15, wherein said extraction electrode is a part of said photosensitive drum.